Several sites are saying it's supposed to launch at $249-259. Performance wise, it needs to be between the 5830 and 5850, but who knows what it'll be. It's almost shame GF104 is rumored to be topping out at 256 shaders, as it'll likely be able to clock higher than GF100 and therefore have better performance (for games) per shader than GF100.
Hopefully the fall refresh will cut some of the gpgpu fat off the consumer GF100 part so the dies are smaller, draw less power per mhz, can be clocked higher, and use all 512 cores. Here's hoping.